Inverter circuit with a control circuit for leading transistors more effectively into a turned-off state

ABSTRACT

An inverter circuit, comprising a control circuit for improving the switching off of transistors (1 and 2). The control circuit includes a secondary winding (16) for an inductive winding (7) included in a load circuit, said secondary winding being connected in series with a winding (17) for a base control transformer (3) of transistors (1 and 2). The control circuit closes itself by way of a diode (21 or 20) and transistor (1 or 2). Thus, a voltage induced in secondary winding (16) indicating the rate of decrease of the collector current of transistor (1 or 2) improves or accelerates the switching off of transistor (1 or 2) when the hazard of simultaneous conduction of transistors increases e.g. as a result of the reversal of loading state or variation of individual characteristics (such as storage time) of transistors.

The present invention relates to an inverter circuit, comprising:

two transistors connected in series across the terminals of adirect-current supply

a control transformer for transistors, the secondary windings of saidtransformer being connected to transistor base control circuits,

a load circuit including a series connection of inductive winding andcapacitor and being connected across a point between transistors and apower source, and

a base current control circuit, including the inductive winding'ssecondary winding and the base control transformer's winding, connectedto said secondary winding.

This type of inverter circuit is anticipated in U.S. Pat. No. 4,045,711.In that publication, the base current of transistors is controlledprimarily by means of the secondary winding of a resonance circuit'sinductive winding in a manner that a base current phase shifts forwardrelative to the collector current of a transistor, bringing oftransistors into a conductive state being delayed so that the current ofone transistor has had time to cut off despite the storage time. Thiseliminates the simultaneous conducting of transistors and power lossescaused thereby.

Occurring in practice, however, are the following variation factorswhose action is not compensated for by this prior art circuit:

1. The frequency and/or amplitude of a resonance circuit current mayfluctuate considerably with varying load. As the frequency and/oramplitude increases, the hazard of simultaneous conducting oftransistors increases. In order to prevent this, the leading of atransistor into a turned-off or non-conductive state should be made moreeffective or sped up, but this cannot be achieved by the prior artcircuit switching.

2. The individual properties of transistors vary considerably. Forexample, the voltage drops occurring across different transistors withthe same base current are unequal. When the voltage drop is minor, atransistor is deep in saturation and can be slowly led into anon-conductive state, in other words the "storage time" of a transistorwill be long. The prior art circuit switching does not at all observethis individual variation of the properties of transistors, which is whytransisotrs must be selected carefully if simultaneous conductivity oftransistors is to be avoided.

An object of the invention is to provide an inverter circuit whichincludes a base current control circuit for observing both the state ofa load and the state of a transistor.

This object is achieved by means of an inverter circuit of the inventionwhose characteristic features are set forth in the annexed claims.

One practical embodiment of the invention will now be described in moredetail with reference made to the accompanying drawings, in which

FIG. 1 shows a circuit diagram of an inverter circuit of the inventionapplied as a ballast for a discharge lamp and

FIG. 2 shows a voltage division diagram of a base current controlcircuit, the respective voltages being indicated in FIG. 1 in thesituation where transistor 1 is conductive.

Aside from a novel base control circuit, a corresponding invertercircuit and its operation has been disclosed in the Applicants' FIPatent specification No. 63147. However, the structure and operation ofan inverter circuit will now be briefly explained. Across thedirect-current terminals + and - are connected a filtering capacitor Cas well as two in series connected transistors 1 and 2, geared inalternating phase operation by means of a base control transformer 3.Secondary windings 5 and 6 of said base control transformer 3 areconnected to the bases of transistors 1 and 2 in a manner that the basesof each transistor receive opposite phase control voltages relative toeach other. Thus, when one transistor is conductive the other isnon-conductive and vice versa.

Across a point between transistors 1 and 2 and a power source isconnected a load circuit, comprising an inductive winding 7 andcapacitors 10 and 11 connected in series therewith, the current flowingthrough said capacitors alternately on successive half-cycles. Connectedin series with a series resonance circuit provided by winding 7 andcapacitors 10 and 11 is a connection in parallel consisting of a lamp 8and an ignition capacitor 9. Resonance capacitors 10 and 11 areaccompanied by stabilizing diodes 23 and 24 which restrict a voltageacross capacitors 10 and 11 in a manner that the voltage will bestabilized at point 22.

Protective diodes 14 and 15 provide the current of inductance 7 with aflow path when both transistors 1 and 2 are in non-conductive state.Resistors 12 and 13, connected to the base-emitter circuit oftransistors 1 and 2, serve to damp undesired oscillations in a per seknown manner.

The base current control circuit includes a secondary winding 16 forinductive winding 7, said secondary winding being connected in serieswith a winding 17 of base control transformer 3. When transistor 1 is inconductive state, the base current control circuit closes itself througha diode 21 and the collector-emitter circuit of transistor 1.Accordingly, when transistor 2 is in conductive state, the controlcircuit closes itself through a diode 20 and transistor 2. Whilst one ofthe diodes 20, 21 closes the circuit of said control circuit, the otherdiode will prevent the current from flowing through the presentlyconductive transistor and control circuit to the power source terminalwhich is opposite relative to the presently conductive transistor.

A control circuit series resistor 18 limits the control current toproper strength.

A capacitor 19 is not absolutely necessary but it provides the followingaction: With transistor 1 or 2 turning non-conductive, current peakproduced by the circuit inductance strives to pass through diode 20 or21 in wrong direction. In order to eliminate the effect of this currentpeak on a current transformer made up by windings 17 and 5, said peak ispassed through capacitor 19.

The operation of a base current control circuit proceeds as follows. Itis presumed that transistor 1 is in conductive state. The collectorcurrent of transistor 1 begins to fall in this example at a falling ratedetermined by the resonance frequency of a load circuit. Falling of thecurrent in winding 4 results in the corresponding fall of a base currentflowing through winding 5. When the base current falls down to zero andturns negative, the transistor still remains conductive for a shortperiod due to the stored charge carriers. As the base current grows innegative direction until the charge carriers are eliminated, thetransistor will be switched off. In order to switch off transistor 1before transistor 2 turns conductive, the base current control circuitfunctions as follows. A voltage U₃ inducing in winding 16 is dependanton the falling rate of the collector current of transistor 1, selectedto be conductive in this study. When the falling rate or rate ofdecrease is sufficient to generate voltage U₃, which together withvoltage U₂ exceeds the voltage drop U₁ of transistor and the biasingvoltage of diode 21, a control current begins to flow in the controlcircuit. The control current generated by voltage U₃ loads by way of acurrent transformer made up by windings 17 and 5/6 a control transformer3 in a manner that the decrease, reversal of the forward base current oftransistor 1 and backward increase will be sped up, the elimination ofcharge carriers or transmitters from the transistor being sped upaccordingly, and the transistor can be switched off more quickly.Essential in this respect is also that the base current is not actedupon until in the collector current decrease phase, whereby thetransistor losses shall not increase due to the fact the transistorwould be too early switched out of the saturation where its voltage dropis at minimum. It will be appreaciated further that the circuitautomatically observes the state of a load since, with the frequency oramplitude increasing, the rate of decrease of the collector currentincreases accordingly and the voltage U₃ inducing in secondary winding16 increases thus accelerating the switching of transistor out of thesaturation during the collector current decrease phase.

Since the collector-emitter circuit of a transistor is part of thecontrol circuit, it can be noted from the voltage diagram of FIG. 2 thatthe control automatically observes differences in the characteristics ofindividual transistors. Since the winding voltage U₃ is in series withthe collector-emitter voltage U₁ of a given conductive transistor 1 or2, said voltage U₁ decreases control voltage U₃ the more the higher saidcollector-emitter voltage U₁ is. On the other hand, if U₁ is low andtransistor is deep in saturation and switched off slowly, voltage U₁will decrease control voltage U₃ less, the latter thus generating ahigher control current which in turn increases the load on transformer 3for decreasing and reversing and/or increasing in negative direction ata faster rate.

On the following half-cycle, with transistor 2 conducting and transistor1 in non-conductive state, the operation of said control circuit isexactly the same, only the direction of control voltage U₃ +U₂ will bereversed and the control circuit closed through transistor 2 and diode20.

It will be appreciated that achieved by a simple circuitry is a controlwhich automatically observes the variations appearing in the state of aload or individual characteristics of transistors. Thus, no additionalcircuits are required for observing the state of load in control andtransistors can be cheaper, not particularly selected transistors.

Although the invention has been described applied in an inverter circuitfor the ballast of a dischage lamp, it is obvious that the invention canbe applied in all inverter circuits regardless of the type of load.Neither is the invention limited to the case where a base controltransformer is fitted with a primary winding 4 connected with a loadcircuit but, instead, the control for secondary windings 5 and 6 of abase control transformer can be applied as a positive control from anexternal control source, the circuit not being freely oscillating. Acontrol circuit improving the switching off of transistors operates inthis case equally well.

I claim:
 1. An inverter circuit, comprising:two transistors (1, 2)connected in series across the terminals of a direct-current supply, acontrol transformer (3) for transistors, the secondary windings (5, 6)of said transformer being connected to base control circuits oftransistors (1, 2), a load circuit including a series connection of aninductive winding (7) and a capacitor (10, 11) and being connectedacross a point between transistors (1 and 2) and a power source, and abase current control circuit, including a secondary winding (16) forsaid inductive winding (7) and a winding (17) for a base controltransformer (3), said latter winding being connected to said secondarywinding (16), characterized in that the base current control circuitfurther comprises a collector-emitter circuit for transistor (1, 2) andthat the control circuit is connected to the terminals of a power sourceby way of two rectifiers (20, 21), a given one of said rectifiersclosing the control circuit while the other rectifier prevents thecurrent from flowing through a presently conductive transistor (1 or 2)and control circuit (16, 17) to the power source terminal (- or +)opposite to said presently conductive transistor, whereby a voltage(U₃), induced in secondary winding (16) of said inductive winding andindicating the rate of decrease of the collector current of transistor(1 or 2), said voltage (U₃) being decreased by a collector-emittervoltage (U₁) of transistor (1 or 2), improves the switching off oftransistor (1 or 2) as the hazard of simultaneous conduction oftransistors (1 and 2) increases.
 2. An inverter circuit as set forth inclaim 1, characterized in that the base current control circuitcomprises a series connection, including a secondary winding (16) forinductive winding (7), a winding (17) for base control transformer (3),a rectifier (21 or 20) and a transistor (1 or 2) as well as a resistor(18) determining the strength of a control current.
 3. An invertercircuit as set forth in claim 1 or 2, characterized in that the basecurrent control circuit is connected to a point between diodes (20 and21), connected in series and in backward direction across the terminalsof a power source.